The seller seems to be making a lot of claims about this guitar without any facts to back them up. No serial number anywhere. How then does he conclude that it was likely made in 1976 and is most probably a prototype?
Despite a good track record in terms of feedback I would always want to know precise facts if I was paying the kind of money that this seller is wanting. Remember, there will always be other guitars to buy. I am no expert on this however, there will be others here who can better advise you. I have been around ebay however and have learnt to avoid purchases unless I am 100% satisfied.